Which metal will spontaneously react with Zn2 + (aq), but will not spontaneously react with Mg2 + (aq) ?

(1) Mn (s) (2) Cu (s)

(3) Ni (s) (4) Ba (s)

This refers to a displacement reaction so we simply look at what can displace Zinc and not Magnesium so it will be Barium which is more reactive than Zinc but less reactive than Magnesium.

Correct Answer: option (1) Mn (s)

Reason:

The spontaneity of electrochemical cell, depends on the it's Eo value. Electrochemical cells with positve Eo are spontanous and vice-versa.

In present case, the Eo of half-cell of interest are as follows:

Eo Zn2+/Zn = - 0.763v

Eo Mg2+/Mg = 2.37v

Eo Mn2+/Mn = - 1.18v

Therefore, Eo cell (with Zn as one of the half-cell) = Eo Zn2+/Zn - Eo Mn2+/Mn

= - 0.763 - (-1.18)

= 0.417v

On other hand, Eo cell (with Mg as one of the half-cell) = Eo Mg2+/Zn - Eo Mn2+/Mn

= - 2.37 - (-1.18)

= - 1.19v

Thus, Mn (s) metal will spontaneously react with Zn2 + (aq), but will not spontaneously react with Mg2 + (aq)